A great natural anchorage in the Orkney Islands, Scapa Flow is bordered by the islands of Orkney Mainland, Hoy, Burra, Flotta and South Ronaldsay and extends for 10 miles (16 km) east to west and 8 miles (13 km) north to south. Scapa Flow has played an important role in naval history. It was a key base for the British Navy in both World Wars and the British Grand Fleet sailed from here to the Battle of Jutland in 1916. Disaster struck in 1917 when HMS Vanguard exploded at anchor in the Flow in 1917, with the loss of over 1000 lives, reputedly due to the ignition of unstable cordite. It was here too that the remainder of the German High Seas Fleet was scuttled in 1919, having surrendered the previous year. This sinking caused widespread environmental damage and, although most of the ships were raised for salvage, seven remain where they sank and these wrecks are popular with scuba divers.
During the Second World War, Scapa Flow was used by the Home Fleet as a base for its role in protecting the Arctic Convoys. After the sinking of HMS Royal Oak in Scapa Flow by a German U-boat, with the loss of 833 lives, Prime Minister Winston Churchill ordered the creation of a defence along the eastern side of the Flow to prevent a further attack. The result, known as the Churchill Barriers, comprised a series of causeways, which were built by Italian POWs to link the islands of South Ronaldsay, Burray, Glimps Holm, Lambs Holm and the Orkney Mainland. The Royal Oak still lies a half-mile (1 km) offshore, to the south of Scapa Bay, as a designated war grave.
The naval headquarters was at Lyness on Hoy, and there are considerable remains of the shore station and oil terminal there, together with a museum - the Scapa Flow Visitor Centre.